Cochlear implants can provide hearing to profoundly deaf or severely hearing impaired persons. Unlike conventional hearing aids which mechanically apply an amplified sound signal to the middle ear, a cochlear implant provides direct electrical stimulation to multiple stimulation electrodes that excite the acoustic nerve in the inner ear. Most existing cochlear implant stimulation coding strategies represent a sound signal by splitting it into distinct frequency bands and extracting the envelope (i.e., energy) of each of these bands. These envelope representations of the acoustic signal are used to define the pulse amplitude of stimulation pulses to each electrode.
The number of band pass signals typically equals the number of stimulation electrodes, and relatively broad frequency bands are needed to cover the acoustic frequency range. A typical acoustic signal such as a human voice producing a vowel includes a fundamental frequency and additional harmonics that are multiples of the fundamental frequency. So if the fundamental frequency is typically between 100 and 200 Hz, then there will also be frequency harmonics that are spaced every 100 to 200 Hz. In existing cochlear implant systems, the band pass filter band widths are usually more than 100 Hz, so that more than one harmonic is usually processed by each band pass filter.
In modern fine structure coding strategies, the stimulation signal timing is derived from the filter bank band pass signals. When multiple harmonics fall within a given frequency band, the derived stimulation timing is usually not representative of any particular harmonic but instead depends on the relative amplitudes and frequency spacing. This means that the stimulation timing in low-to-mid frequency channels is relatively complex instead of simply coding the periodicity of the dominant harmonics. Usually one specific harmonic dominates a filter band, and in normal hearing such a dominant harmonic masks the neighboring harmonics and carries the audio information that should be tonotopically and temporally correct.
In psychoacoustic pitch testing, both periodic pitch and tonotopic pitch concepts have been demonstrated to work for cochlear implant patients. A gradual shift of the stimulation pattern from an apical electrode towards a more basal one at relatively high rates leads to an increase in pitch percept. Nobbe et al. (Acta Oto-Laryngologica, 2007; 127: 1266-1272; incorporated herein by reference) showed that either simultaneous or sequential stimulation leads to just noticeable differences in pitch changes of down to one semitone. Similar results can be found if the low stimulation rate of one electrode is increased, in which case, just noticeable differences in pitch range to within one semitone. These results suggest that a combination of both types of pitch cues could lead to better pitch perception in cochlear implant users. But in existing cochlear implant systems, both tonotopic and periodic pitch cues are not integrated at the same time. The temporal fine structure of the input signal is analyzed in relatively broad bands, and this generates shifts in stimulation patterns at transitions between analysis filters which can lead to unexpected and unwanted changes in pitch percepts.
One coding strategy that partially addresses the above is the Fine Structure Processing (FSP) strategy used in the Med-El OPUS 1 and OPUS 2 speech processors. The FSP strategy codes very low frequency harmonics, usually the fundamental frequency and the second harmonic, by using a filter bank that ranges down to below the expected fundamental frequencies. The spacing of the lowest frequency bands is such that the harmonics coded are usually resolved, that is, only one harmonic falls into one low frequency filter band. But higher harmonics are not explicitly resolved by this type of signal processing. In addition, the shift of harmonics is mainly coded temporally. A tonotopic shift of the temporal code of fundamental frequency gliding from 100 Hz up is only achieved at around 200 Hz.
The HiRes 120 strategy of Advanced Bionics Corporation uses active current steering and additional spectral bands. The input signal is filtered into a large number of spectral bands and a fast Fourier transformation (FFT) algorithm is applied for fine spectral resolution. Hilbert processing derives temporal detail from the signals while the spectral maximum for each electrode pair is determined across all the filter bands. Pulse rate and stimulus location are determined from the estimated frequency of the spectral maximum. A number of spectral bands are assigned to each electrode pair and the spectral bands are delivered to locations along the electrode array by varying the proportion of current delivered simultaneously to adjacent electrodes in each electrode pair.